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Ion motion becomes relativistic 4 at intensities above 10 22 W cm −2 - an intensity regime demonstrated or anticipated with development projects for very-large-scale lasers like the HERCULES laser at the University of Michigan in the USA, which has generated some of the highest peak intensities ever delivered to a target 5, and ELI 6 (the Extreme Light Infrastructure project), which will create four linked high-intensity laser facilities across Europe. At these intensities, the electromagnetic field drives electrons into relativistic motion, opening the door to useful effects like wakefield acceleration 2 and hard X-ray production by bremsstrahlung, Compton or betatron emission 3. With the advent of chirped pulse amplification (CPA) in 1985 1 came the ability to generate ultrashort laser pulses with intensities in excess of 10 18 W cm −2. The challenge of producing the next generation of particle accelerators, for both fundamental research at laboratories such as CERN and more applied tasks such as proton therapy and nuclear transmutation, has been taken up by the high-intensity laser community.